This invention relates to circuits for controlling the speed of an electric motor and more particularly, to speed controls for use with alternating current motors.
It is well known in the prior art to control the speed of an alternating current motor by periodically interrupting the current flow to the motor so that power is supplied to the motor for some period less than a full cycle. A well known arrangement for accomplishing this method includes a silicon controlled rectifier or other controllable current conducting means in series with the motor and the alternating current source. Timing circuits are typically provided to trigger the silicon controlled rectifier or other controllable current conducting means at a selected phase angle within each half cycle, thereby controlling the power applied to the motor. It is also a common practice to provide a reactive component, such as a capacitor, as a means for timing the triggering of the controllable current conducting means. In such prior art circuitry the rate of charge of such components provided a reasonably acceptable means for triggering the controllable current device at some definite point during the period of an alternating current cycle.
Such circuits, however, have not been entirely reliable and several problems have existed which affect the operation thereof. In particular, timing capacitors utilized in prior art circuits would on occasion discharge at an inappropriate time and the controllable current conducting device would not function properly. Thus, the power supplied to the motor could be erratic under certain operating conditions.
An early example of a speed control circuit employing this type of technology may be seen in U.S. Pat. No. 3,443,118 issued to A. J. Mortimer. A much more complex circuit which attempted to solve the reliability problems experienced in such simple speed control circuits may be seen in U.S. Pat. No. 3,739,249 issued to G. M. Rosenberry, Jr. The Rosenberry, Jr. patent utilizes a complex charging mechanism to attempt to insure reliability of the action of the timing capacitor.